1. Field of the Invention
This invention relates in general to the field of pressurized light water nuclear reactors and in particular to the radial neutron reflector surrounding the nuclear core for improved neutron economy.
2. Description of the Prior Art
It is well known that commerical, pressurized light water, nuclear reactors are both a technical and commercial success. In such reactors, a reactive region commonly referred to as a nuclear core contains nuclear fuels such as uranium 235, as well as other fissile materials, which undergo sustained fission reactions and in so doing, generate heat. There are, of course, other materials in the nuclear core, the presence of such other materials, however, is not germane to this invention and, accordingly, will not be discussed. A group of mechanical components, which are known as reactor internals, structurally support the core within a hermetically sealed pressure vessel. The reactor internals also direct the flow of a cooling medium, such as light water, into the pressure vessel through the nuclear core, and out of the pressure vessel. The cooling medium which is alternatively called the reactor coolant, removes the heat generated by the fissioning of the nuclear fuel and transfers the heat to another cooling medium within heat exchangers which are located external of the pressure vessel. The second cooling medium, which also is usually water, is converted into steam in the heat exchangers and is thereafter used to produce electricity by conventional steam turbine-electrical generator combinations.
The reactor internals usually include a core barrel comprising an elongated cylinder which is interposed between the nuclear core and the cylindrical wall of the pressure vessel. The nuclear core then is positioned within the core barrel. Typically, the reactor coolant enters the pressure vessel through one or more inlet nozzles, flows downward between the pressure vessel and the outside of the core barrel, turns 180.degree., and flows upward through the core and through the space between the outside of the core and the inside of the core barrel. The heated reactor coolant then turns 90.degree. and exits the pressure vessel through one or more exit nozzles and then to the heat exchangers previously mentioned.
In the pressurized water reactors, such as the one described, the fissioning of the nuclear fuel results from the capture of a neutron by the nucleus of the atoms of the nuclear fuel. It is well known that each neutron producing a fission causes heat and the production of more than one other neutron (on the average, 2.1 neutrons are released per capture). To sustain the nuclear chain reaction, at least one of the newly produced neutrons must then fission another atom of fuel. Since the neutrons generated are fast neutrons, and fissioning is enhanced by slow neutrons, it is advantageous that the fast neutrons be slowed down or thermalized within the confines of the nuclear core. The reactor coolant comprising light water is an excellent moderator of neutrons; hence, in reactors primarily using U-235 as the nuclear fuel, it is the primary means by which the fast neutrons produced by the fission process are thermalized or slowed down so as to increase the probability that another fission may occur and thereby sustain the chain reaction. The excess neutrons are accounted for in a number of different ways. Some are absorbed by the reactor internals. Others are slowed down and absorbed by a nuclear poison such as boron which is dissolved in the primary coolant. Other neutrons are absorbed by load follow control rods containing nonburnable control poisons which control rods comprise the means for controlling the nuclear reactor. Others are absorbed by special control rods which are interspersed throughout the nuclear core and made of materials specifically selected to absorb neutrons such as burnable poisons which as their name implies are burned during reactor operation and, therefore, become less effective in proportion to the continually reducing reactivity of the nuclear core. Still other neutrons are absorbed by poisons which build up within the nuclear fuel and are caused by the fission process itself. Quite obviously, the accounting for the excess neutrons is a complicated matter which can, however, be summarized by stating that some excess neutrons are purposefully absorbed while the remainder are inadvertently absorbed. And, it is desirable to reduce the number that are inadvertently absorbed.
In order to extend the life of the nuclear core as long as is practical so as to minimmize time consuming reactor shutdowns for refueling purposes, the fuel assemblies are provided with enriched nuclear fuel, usually enriched uranium 235. This excessive amount of reactivity is designed into the core at startup so that as the reactivity is deleted over the life of the core, the excess reactivity is then used, thereby extending the life of the core. The amount of enrichment continuously decreases as the reactor operates until such time as the core can no longer sustain the chain reaction. Then the reactor must be shut down and refueled. During the initial stages of reactor operation or during the phase which is known as beginning of life, special neutron absorbing control rods may be inserted within the core and/or additional soluble poisons are dissolved within the reactor coolant and/or burnable poisons may be included within the fuel assemblies to absorb the excess reactivity. As the excess reactivity decreases due to the nuclear fuel being burned, the amount of insertion of the special control rods and/or the amount of soluble poison and/or the burnable poisons within the special control rods and/or the fuel assemblies are consumed consistent with the reduction in excess reactivity to maintain the chain reaction. In this manner, the excess reactivity is held in abeyance until it is needed.
Enriched uranium is extremely expensive. It is preferably, therefore, to reduce the amount of enrichment whenever possible but without reducing the extended operating length of the life of the core. One recognized method or theory to accomplish this result is by making more efficient use of the neutrons produced by the fission process. An area where present day nuclear reactors are relatively inefficient, as regards neutron economy is concerned is, in general, the region of the reactor between the pressure vessel and the nuclear core, and in particular between the internal diameter of the core barrel and the outer periphery of the fuel assemblies. Since the fuel assemblies are square in cross section, side-by-side stacking of the fuel assemblies produces an irregular non-circular outer periphery of the core. In the prior art, stainless steel vertical plates are positioned against the irregular periphery of the core. The vertical plates are supported by a plurality of horizontal "former plates" bolted to the vertical reflector plates. The former plates are in turn bolted to the core barrel.
The former plates are specially shaped to provide for the transition from the irregular core periphery to the circular shape of the core barrel. The vertical plates provide for core lateral support and prevent the reactor coolant from bypassing the core. Although not necessarily originally intended, it has been determined that the vertical stainless steel plates also provide a radial neutron reflection function. In this manner, means have been provided whereby some of the excess neutrons produced by the fission process and some of which would otherwise radially escape from the core are reflected by the stainless steel plates back into the core. Unfortunately, the present day design of the stainless steel plates as regards the radial reflector function is not as efficient as desired. For example, the space between the vertical stainless steel and the horizontal former plates is occupied by primary coolant which allows for removal of the heat absorbed by the core barrel, the former plates and the vertical plates. While water is an excellent moderator it is an inefficient reflector. Thus, while some neutrons are reflected back into the core, a great number are thermalized and/or absorbed by the relatively large volume of reactor coolant located radially external of the nuclear core.
Patent Application Ser. No. 576,655 entitled "Radial Neutron Reflector" filed Feb. 3, 1984 by Freeman, et al., and assigned to Westinghouse Electric Corporation, disclosed means to overcome the problems of the reflector region of the prior art. In that patent application, the space between the core periphery and the core barrel is divided into a number of different shapes which are filled with solid stainless steel plates or within which are placed either a plurality of elongated circular metal rods or blocks of metal stacked on top of each other. Reactor coolant flows between the circular rods or through special holes in the stacked blocks or solid steel plates in order to cool the material surrounding the core which is intended to act as a neutron reflector. By substantially removing the water from the reflector region and replacing it with a material other than hydrogen, the prior art problem of loss or absorption of the radially escaping neutrons was overcome. The radially escaping neutrons are bounced or reflected back into the reactive core region where they may be effectively and advantageously used. While the noted pending patent application comprises a substantial contribution toward effective management of radially escaping neutrons, there is the desirability of improving upon this contribution. Accordingly, a major object of the present invention is to provide apparatus designed to be used in the reflector region of a nuclear reactor which improves upon the type of reflector disclosed in the referenced patent application.
In filling the space between an irregular core periphery and a circular barrel surrounding the core, it is difficult if not impossible to utilize a single standard shape and provide uniform cooling. An irregularly shaped annulus inherently requires a number of specially fitted shapes to fill the myriad of oddly angled corners and spaces left over from stacking of the regular shapes. Cooling of the irregular and the regular shapes is important to prevent thermal bowing and excessive thermal stresses. Proper cooling requires properly sized coolant holes and coolant channels between the shapes. Cooling of both the regular and the irregular shapes is, however, problematic because of the equally irregular shape of the interstitial spaces between the shapes, which are due primarily to the fitup of the shapes. Too large of a space causes overcooling; while too small of a space results in overheating and perhaps failure. Providing properly sized coolant holes or channels in the regularly shaped reflector pieces is a problem because, in part, of the nature of the most effective reflector material--zirconia. Zirconia cannot be directly exposed to the reactor coolant and must be clad or covered with a protective material such as stainless steel or zircaloy. Hence, the cooling holes must be internally fitted with tubes of such protective materials. However, in using internal cladding, fitup problems and differential thermal expansion problems usually arise. External cladding of the zirconia shapes is problematic for these same reasons. In order to fit the blocks of the zirconia into the external cladding, a clearance space must necessarily exist between the outer dimensions of the blocks and the inner diameter of the cladding. After assembly, the clearance space still exists. Pressure within the reactor vessel during reactor operation subsequently causes the cladding to collapse onto the blocks (the cladding prevents pressure equalization) and in so doing a number of wrinkles undesirably appear in random locations. These wrinkles interfere with the carefully designed cooling flow between the shapes resulting in the aforementioned problems.
Accordingly, another object of the present invention is to provide apparatus for use as a radial neutron reflector which allows side-by-side stacking of clad shapes with substantially no gap between adjacent shapes.
Another object of the present invention is to provide clad reflector elements which allow side-by-side stacking and have no clearance between the inner reflector material and the outer cladding.
Another object of the present invention is to provide clad reflector elements which have a wrinkle-free clad surface.
Another object of the present invention is to provide clad reflector elements which have accurately sized and positioned coolant flow passages.
Another object of the present invention is to provide clad reflector elements which have a high solid-to-void ratio when stacked together in the reflector region of the core.
Other objects resulting from the present invention, although not specifically listed, are intended to be within the scope of the present invention. Accordingly, the above-stated objects are not intended to be a complete listing of all the objects or disadvantages of the present invention.